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First principles density functional theory has been used to calculate the 2-D band structure of Si slabs with different thicknesses. From the calculated 2-D band structure, electron longitudinal and transverse effective masses have been extracted as a function of the slab thickness. These thickness-dependent electron effective masses have then been used to simulate I D-VG characteristics of scaled, sub-10 nm double-gate (DG) MOSFETs and to compare them with the results obtained using bulk masses. The channel thickness dependence of the Si band structure starts to affect noticeably DG MOSFET performance at channel lengths below 10 nm, lowering the on-current by approximately 10, for transistors with a body thickness of 2.6 nm, and by 20, for transistors with a body thickness of 1.3 nm. On the other hand, the subthreshold swing is improved by 10, in the 6-nm-gate length DG MOSFET and by 15, in the 4-nm-gate length device. Finally, the impact of thickness-dependent effective masses has been related to the behavior of the transmission coefficients.